Save edited buffers before running a task

Introduces a new task field for configuring which buffers are saved. For
now, this defaults to saving all buffers, but in the future we could
have a global task template to configure this setting for dynamically
created tasks.

Needed for #10251.

Release Notes:

- Edited buffers are now saved before running a task. This can be
configured with the new "save" field in `tasks.json`.

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Co-authored-by: Kirill Bulatov <mail4score@gmail.com>